Spacecraft in orbit about the earth, such as artificial satellites, require means for attitude determination in order to maintain a desired three axis (roll, pitch and yaw) orientation relative to the earth.
Earth sensors are devices that are mounted to satellites for sensing the infrared radiation emitted by the earth and for producing a signal from which two axis attitude information (roll and pitch) of the satellite relative to the earth can be determined. Orientation of around the yaw axis is usually determined by other means, such as gyroscope, or the observation of other heavenly bodies.
Conventional earth sensors use an electromechanical mechanism, such as a chopper or scanning mirror, in combination with single element or linear array infrared detectors. Such scanning type earth sensors suffer from many disadvantages. First, in view of the electromechanical chopper or scanning mirrors are heavy and expensive to build and they are difficult to align and adjust. Further, the resonant frequencies of the known electromechanical scanning mechanisms react with spacecraft vibrations to cause error in the attitude determination. Further still, the single element infrared detectors used in such scanning type sensors are also expensive and they have a tendency for high incidence of qualification failure.
Another type of earth sensor uses thermopiles as the detective source for "staring" at the earth. Each thermopile consists of many individual thermal couples and the thermal couples are sensitive to temperature variation in the detector environment and between adjacent detectors.
In addition, accuracy of such starring type earth sensors is compromised by the limited amount of image information that the detectors generate due to their small field of view of the earth's limb.
A bolometer is a thermal detector in which the incoming radiation heats the detector, resulting in a temperature rise that is then sensed as a change in the element resistance. Recent advances in infrared detector technology incorporating silicon manufacturing processes have seen the practical development of imaging devices known as microbolometer detectors. A microbolometer detector is a two dimensional area array detector that has three very attractive features: a cost advantage inherent in its silicon manufacturing processes, the capability for chopperless dc coupled sensor, and the performance of microbridge detector structure. The microbridge structure of the microbolometer focal plane allows for much improved performance over conventional IR imagers since the microbridge can be fabricated with very low thermal conductance between pixels which eliminates thermal crosstalk between adjacent sensing elements, enabling near detector limited spatial resolution to be achieved. Microbolometer focal plane arrays with an array size on the order of 320.times.240 pixels have been fabricated directly on silicon read out electronics and have been demonstrated to achieve very sharp and crisp imagery characteristics.
To the inventors' knowledge, there are no known earth sensors which utilize the above-described microbolometer sensor technology for determining attitude information for orbital spacecraft. An earth sensor utilizing such microbolometer sensor technology and that also includes an input optics configuration and image processing means specially adapted to make optimum use of the full and crisp image detection capability of the microbolometer focal plane array would constitute a significant advance in the art.